This invention relates generally to electrophotography, and more particularly to a charging device in which the strength of the corona discharge from a suitable corona generating device may be partially varied to obtain a desired charge on an ion receiving surface.
As is well known in the art, the electrostatic photography process is achieved by uniformly charging a photoconductive insulating layer provided on an electrically conductive support member wherein the layer is treated as a photosensitive layer, exposing the photoconductive layer to an image, and, then, effecting toner development of the image.
During the charging step, the support member has to be grounded. However, it is difficult to obtain a good grounded condition when the support member is formed of flexible material. For example, where the support member is formed of a plastic film or paper made of material such as polyester, triacetate, cellophane, or resin coated paper, it is very difficult to obtain uniform charging of the photosensitive layer. When it is desired to charge a photosensitive layer provided on a support member of the type noted above, it is common in the art to ground the peripheral portions of the support member prior to the commencement of the charging operation. However, the center portion of the member is not grounded thereby decreasing the charge that is obtained on the portion of the surface of the photosensitive layer contacting the ungrounded center portion of the member. In order to obtain a uniform charge on the surface of the photosensitive layer, it is necessary to provide additional means to supply added charging to the central portion of the layer.
Further, as is well known in the art of electrostatic photography, there occurs what is commonly referred to as the "edge effect" whereby the peripheral portion of an image is developed dark and the center portion is light. This is most noticeable when a relatively wide solid area image is to be developed. Thus, in order to minimize the edge effect, it is desirable to charge the center portion of a relatively wide solid area image to a greater level when compared to the remaining portions, to thereby obtain a copy of substantially uniform contrast.